I've actually been looking for this for a long time. I like to travel around and often wonder what will happen to my digital legacy (code, internet bank accounts, subscriptions to all kinds of things, scanned documents). My previous solution was to e-mail an encrypted file to my parents and give them the password to it on a note, which was then put in the safe. I'll try the site but I'm a bit wary of whether or not it actually works. After all, if you disappear off the face of the planet then you won't be able to check that it actually works.
Edit: it also seems like a perfect target for hacking, as people may write information there that evil people could profit from greatly.
I would recommend that everyone encrypts the emails that go on the service, as obviously we can't do it ourselves (we have to actually be able to read them to send them out!).
I guess you create an email, encrypt it with a 2048 bit AES key. And set it up to mail to your 'back up'.
Then you go and create 32 throw away email accounts and have each of them running their own aliveness test on a TOR proxy which is mailing you their queries.
When you go 'off line' the 32 mail their part of the key to the recipient, DMS mails them the main letter. :-)
Not a serious suggestion but it would be fun as a plot device in some cyber fiction.
Edit: it also seems like a perfect target for hacking, as people may write information there that evil people could profit from greatly.